A Critical Role for Innate Immunity In Allogeneic Hematopoietic Cell Rejection Via the TLR4/TRIF Pathway

Abstract 77

Adaptive immunity, especially T cells, has long been believed to be the dominant immune barrier in allogeneic transplantation. Targeting host T cells significantly reduces conditioning for bone marrow cell (BMC) engraftment. Innate immunity has been recently shown to pose a significant barrier in solid organ transplantation, but has not been addressed in bone marrow transplantation (BMT). Using T cell deficient (TCR-β/δ^−/−) or T and B cell deficient (Rag^−/−) mice, we found that allogeneic BMC rejection occurred early before the time required for T cell activation and was T- and B-cell independent, suggesting an effector role for innate immune cells in BMC rejection. Therefore, we hypothesized that by controlling both innate and adaptive immunity, the donor BMC would have a window of advantage to engraft. Survival of BMC in vivo was significantly improved by depleting recipient macrophages and/or NK cells, but not neutrophils. Moreover, depletion of macrophages and NK cells in combination with co-stimulatory blockade with anti-CD154 and rapamycin as a novel form of conditioning resulted in 100% allogeneic engraftment without any irradiation and T cell depletion. Donor chimerism remained stable and durable up to 6 months. Moreover, specific Vβ5½ and Vβ11 clonal deletion was detected in host CD4⁺ T cells in chimeras, indicating central tolerance to donor alloantigens.

Whether and how the innate immune system recognizes or responds to allogeneic BMCs remains unknown. Toll-like receptors (TLRs) are a class of proteins that play a key role in the innate immune system. The signaling function of TLR depends on intracellular adaptors. The adaptor MyD88 transmits signals emanating from all TLR, except TLR3 while TRIF specifically mediates TLR3 and TLR4 signaling via type 1 IFN. To further determine the innate signaling pathways in allogeneic BMC rejection, B6 background (H2^b) MyD88^−/− and TRIF^−/− mice were conditioned with anti-CD154/rapamycin plus 100 cGy total body irradiation and transplanted with 15 × 10⁶ BALB/c (H2^d) BMC. Only 33.3% of MyD88^−/− recipients engrafted at 1 month, resembling outcomes for wild-type B6 mice. In contrast, 100% of TRIF^−/− mice engrafted. The level of donor chimerism in TRIF^−/− mice was 5.1 ± 0.6% at one month, significantly higher than in MyD88^−/− and wild-type B6 controls (P < 0.005). To determine the mechanism of innate signaling in BMC rejection, we examined whether TRIF linked TLR3 or TLR4 is the key pattern recognition receptor involved in BMC recognition. To this end, TLR3^−/− and TLR4^−/− mice were transplanted with BALB/c BMC with same conditioning. None of the TLR3^−/− mice engrafted. In contrast, engraftment was achieved in 100% of TLR4^−/− mice up to 6 months follow up. Taken together, these results suggest that rejection of allogeneic BMC is uniquely dependent on the TLR4/TRIF signaling pathway. Thus, our results clearly demonstrate a previously unappreciated role for innate immunity in allogeneic BMC rejection. Our current findings are distinct from prior reports demonstrating a critical role of MyD88 in rejection of allogeneic skin grafts and lung, and may reflect unique features related to BMC. The findings of the role of innate immunity in BMC rejection would lead to revolutionary changes in our understanding and management of BMT. This would be informative in design of more specific innate immune targeted conditioning proposals in BMT to avoid the toxicity.